The present invention is related to a motor drive apparatus and method, and an electrically-driven brake apparatus.
Recently, while motors have been employed in various sorts of fields, generally speaking, constructions and control contents as to these motors are made different from each other, depending upon characteristics of objects which should be driven by the motors. There are many sorts of purposes, for instance, high speed rotations of motors are major purposes; precise rotation numbers of motors at low speeds are purposes; magnitude of rotation torque of motors constitutes important aspects; torque of motors at low speeds constitutes important aspects, and the like. Therefore, constructions and controls of motors are selected based upon purposes thereof. For example, rotation speeds of motors may constitute important aspects, whereas magnitude of torque in such a drive region that rotation numbers of motors becomes nearly zero may constitute important aspects.
As the latter example, there is such an electrically-driven brake apparatus having the following structure (refer to, for instance, JP-A-11-257389): That is, while a motor is provided in a brake apparatus of an automobile, rotation power of the motor is converted into depression force along a shaft direction, so that braking force is applied via a friction pad to a rotary disk of the automobile.
As the above-described motors, 3-phase AC motors, or multi-phase AC (Alternating Current) motors such as permanent magnet brushless motors are employed many times, in which AC voltages are used as power supply voltages. The motors use inverter apparatuses including a plurality of switching elements in order to apply the AC voltage thereto. In general, switching elements of inverter apparatuses are ON/OFF-controlled based upon a sine wave pulse width modulation control operation. The sine wave pulse width modulation control operation corresponds to such a method for controlling ON/OFF timing and ON/OFF pulse widths of all of the switching elements in such a manner that output voltages of the inverter apparatus become sine waveforms, while all of these switching elements are continuously turned ON/OFF.
However, in the above-exemplified electrically-driven brake apparatus, since the friction pad is depressed to the rotation disk by converting the rotation power of the motor into the depression force, under such a condition that a depression gap becomes zero and the braking force is maintained, the predetermined constant voltage is outputted while the rotation of the motor remains substantially stopped (such a condition that either rotation angular velocity or rotation number of motor becomes smaller than, or equal to very small value, which involves rotation stopping condition). Such a condition will be referred to as a “motor locking” state.
In this case, when the braking force is applied, the rotation number of the motor is nearly equal to “0 (zero).” However, in order to maintain the braking force, the motor is required to continuously output such a torque whose magnitude is larger than, or equal to the predetermined magnitude. When the motor is rotated, since the inverter output voltages are changed in the periodic manner, heat generations of the switching elements are averaged, whereas when the motor is locked, in order to keep the inverter output voltages to the constant values in accordance with the rotation positions of the motor, such a condition is held that currents are concentrated to flow through a specific element. More specifically, if the currents are concentrated to the specific element, then the temperature of this specific element is increased. As a result, a temperature fail of the inverter apparatus may readily occur. Therefore, there is such a problem that lifetimes of the elements are lowered, and the like.
Also, since there are many limitations as to installation spaces of motor drive apparatus for automobile auxiliary machines, such active cooling methods as water cooling methods and forcible air cooling methods are not employed, but in order to reduce the motor drive apparatus, lowering of loss of the motor drive apparatus constitutes the necessary condition.